By Richard Black
BBC Science Correspondent
The target of giving Aids drugs to three million people in the developing world by the end of 2005 will not be met, a leading expert warns.
The aim of the project is to widen the availability of HIV drugs
Professor Joep Lange, co-chair of the UNAids conference in Bangkok, told the BBC the target was "inflated and unrealistic".
The 'Three by Five' initiative is part of UNAids and World Health Organisation attempts to stem the spread of HIV.
The WHO declined to respond to Professor Lange's comments.
Discussions on the so-called 'Three by Five' initiative began in 2001, and the WHO made it a formal target on World Aids Day last December.
Even then it was widely regarded as ambitious; and now Professor Lange, who's also President of the International Aids Society, says it simply won't be met.
He told the BBC in an interview from Bangkok: "I think it's impossible." .
"It's an inflated target that is totally unrealistic."
By expressing these doubts on Three by Five, Professor Lange is putting on record what many other observers have been saying privately - UNAids itself admits that attempts to enrol HIV-positive people in treatment programmes are lagging.
If Professor Lange's analysis is correct, it means that a major initiative to curb the spread of HIV is failing.
UNAids Executive Director Dr Peter Piot has described Three by Five as "a massive challenge, but one we cannot afford to miss."
Last week's UNAids report on the global epidemic showed that last year, five million people were newly infected with HIV, and three million died from Aids.
Professor Lange says the slow progress is partly down to lack of money - in particular the Global Fund to Fight Aids TB and Malaria, launched to great fanfare three years ago as the international community's definitive response to these developing world diseases, has received far less in donations than it initially hoped.
Ideological tussles between the current US administration and other major players have diverted money, energy and time.
Many countries where there's a great need for anti-retroviral drugs don't have the infrastructure to deliver them reliably.
But Professor Lange was also critical of the concept itself.
"Obviously it's good to get attention; but at the same time we need to make plans that take into account things that are not taken into account now," he told the BBC.
"Just putting a number out and trying to get as many people on therapy as quickly as possible without actually making sure the structures are there to support that I think is not actually the most productive way forward."
Coming from a central figure in the international Aids community, this is an explicit criticism of the UN's strategy.
The WHO declined to offer a response - it is due to release its own progress report on Three by Five at a news conference in Bangkok shortly before the UNAids conference officially opens.